If C can detect that we are in the context of a canvas page, and Facebook already knows that B will not wait more than X seconds, they might as well issue an HTTP 408 Request Timeout or something similar after X seconds, because there's no real point in them giving me the info they aren't going to wait for...

I went ahead and created a Feature Request in Facebook's Bugzilla. You can vote for it if you think it will help you.#5528